The invention relates to a priming valve device that can ensure priming of the pump in a water circuit. The invention further relates to a beverage machine such as a coffee machine comprising such a priming valve device.
In modern coffee machines, beverages can be prepared by passing hot water under a certain pressure through a container or chamber that contains the beverage ingredient. Water is pumped from a water tank by a pressure pump, e.g., a piston pump, that pushes water in a heater for heating, typically a thermobloc, a boiler or an on-demand heater, and injects the hot water through the beverage ingredients. The beverage ingredients are extracted or dissolved under pressure and a beverage extract or mixture is released through the container. The pressure pump may deliver a pressure of more than 5 bar, typically 15 to 20 bar; which pressure depends on the backpressure opposed by the container, the ingredients (e.g., granulometry, bed size, . . . ), a backpressure valve, etc., but also a particular mode of opening of the container and/or by a particular filtering mode of the beverage. Certain containers require a high pressure to be delivered by the pump and the opening for the release of the beverage extract is carried out by tearing of a plastic or aluminium membrane of the container.
Priming problems of the pressure pump occurs typically when the water tank becomes empty, air introduces in the pump and the water tank must be replenished by the user. The pressure differential at the pump becomes too high between the entry, where pressure is at about atmospheric pressure, and the exit of the pump, where the pressure is much higher, and water can no longer be pumped. To resolve this problem, the portion of the water downstream of the pump must be drained which may so require the opening of the beverage chamber and the loss of the container.
Another problem relates to the limestone scaling of the heater. The heater must preferably be maintained under a certain pressure in order to ensure that hot water remains in liquid form. The reduction of pressure leads liquid water to transform into steam which causes serious limestone scaling problems. In particular, heaters such as thermoblocs are very sensitive to scale problems because of the narrow section of tubes in the serpentine heating circuit that becomes clogged more rapidly.
In the prior art, there have been attempts to develop priming valve devices for vibration pumps, in particular, in EP 1 003 997 B1. The device performs the function of self-priming the pump, balancing a negative pressure in a boiler when it cools down and furthermore protecting the boiler against overpressure. The device comprises a non-return or check valve, an air release valve arranged upstream the non return valve to perform the self priming function for the pump and a safety valve for allowing air entrance in the boiler and associated downstream of the check valve and in parallel with the air release valve. Such a device is relatively complicated and may lead to leakage issues because it requires several separate valves to be operated in a coordinated manner; each valve acting upon separate resilient means of different compressive strength. In particular, the device tends to leak when it is used to prime a piston pump. The device is unstable (i.e., it operates very small repeated openings/closings corresponding to the vibration of the pump) because of the water flow fluctuation that occurs at the frequency of the pump. Another problem of this prior art is due to the fact the check valve can only open under the dynamic pressure created by the pump and thus a backpressure of several bars is created by the check valve. As a result, the extraction of the beverage ingredients is performed at lower pressure than the optimum pressure range in absence of the backpressure valve. It is therefore necessary to propose a solution for priming a pressure pump in a water circuit of a beverage machine which is of a simpler and more efficient functioning, i.e., that should reduce leakage problems and the significant pressure loss of the prior art devices.